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What is Context Dependent Symbolic Link

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

When using the shared cluster file system, there is just one copy of the entire directory structure. Just one set of directories exists where the data files, control files, redo log files, and archive log files are located. This is advantageous in many respects. However, sometimes there may be a desire for the relationship between a set of files or a directory and the shared CFS directory to be unique for each node/host within the cluster. For example, the DBA may want to keep a local tnsnames.ora file or a local listener.ora file.

For the purpose of setting up Intelligent Agent, the DBA may want to separate the $ORACLE_HOME/network directory from the shared Oracle home installation without having to physically install the Intelligent Agent on each node in the cluster. In such situations, setting up a context dependent symbolic link (CDSL) creates a node-dependent copy of the file or directory.

There are many cluster file system products that can be used for building the RAC system. They include: Tru64 CFS for HP/Compaq Cluster servers, Veritas CFS for Solaris based RAC Clusters, PolyServe Matrix Server for Linux and Windows based RAC Clusters, and Oracle cluster files systems for Linux and Windows RAC clusters. Some of the important features of these products will be examined next. Only some of these, however, support CDSL.

Veritas CFS

The VERITAS Database Edition/Advanced Cluster for Oracle RAC enables Oracle to use the CFS. The Veritas CFS is an extension of the VERITAS File System (VxFS). The Veritas CFS allows the same file system to be simultaneously mounted on multiple nodes. Veritas CFS is designed with master/slave architecture. Any node can initiate a metadata operation (create, delete, or resize data) and the master node carries out the actual operation. All other (non metadata) I/O goes directly to the disk.

A distributed locking mechanism, called the global lock manager (GLM) is used for metadata and cache coherency across the multiple nodes. GLM provides a way to ensure that all the nodes will have a consistent view of the file system. When any node wishes to read data, it requests a shared lock. If another node wishes to write to the same area of the file system, it must request an exclusive lock. The GLM revokes all shared locks before granting the exclusive lock and informs reading nodes that their data is no longer valid.

CFS is used in DBE/AC to manage a file system in a large database environment. When used in DBE/AC for Oracle-RAC, Oracle accesses data files stored on CFS file systems with the ODM interface. This essentially bypasses the file system buffer and file system locking. This means that only Oracle handles the tasks of buffering data and coordinating writes to files and not the GLM, which is minimally used with the ODM interface.

Using this out-of-band fencing is a significant benefit in large clustered environments where the alternative fencing approach known as STOMITH (shoot the other machine in the head) is neither sufficiently reliable nor acceptable.

HP Tru64 CFS

Tru64 CFS is a layer on top of the Advfs file system. When direct I/O is enabled for a file by opening the file with the o_directio flag, read and write requests on it are executed to and from disk storage through direct memory access, bypassing AdvFS and CFS caching. This improves I/O performance for database applications that do their own caching and file region synchronization.

Oracle uses the direct I/O feature available in CFS. Direct I/O enables Oracle to bypass the buffer cache (no caching at file system level). Oracle manages the concurrent access to the file itself as it does on raw devices. Direct I/O does not go through the CFS server. File creation and resizing is seen as a metadata operation by Advfs and this has to be done by the CFS server. Consequently, file creations and resizes must be run on the node where the CFS server is located. File operations might take longer when the CFS server is remote.

In a TruCluster server, the device request dispatcher subsystem controls all I/O to physical devices. All cluster I/O passes through this subsystem, which enforces single-system open semantics so only one program can open a device at any one time.

The following are some general features of a Tru64 CFS:

* The Cluster File System (CFS) makes all files, including the root (/), /usr, and /var file systems, visible to and accessible by all cluster members. There is a single copy or image for all cluster members.

* A single cluster member serves each file system. Other members access that file system as CFS clients, with significant optimization for shared access.

* Oracle RAC automatically does direct I/O on Tru64 UNIX file system storage. This can significantly improve I/O performance for the database since Oracle9i does its own caching and file region synchronization.


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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